This application claims priority from R.O.C. Patent Application No. 090110463, filed May 2, 2001, the entire disclosure of which is incorporated herein by reference.
FIG. 1 shows a cross-section of a general N-type power IC, the fabrication method of which is described herein. An oxide layer is formed on the N-type substrate 10. A patterned photoresist layer is formed on the oxide layer to define an active region. P type dopants are implanted into the substrate 10. After the drive-in process is proceeded, a Pxe2x88x92type body 12 is formed in the active region. Then the photoresist layer is removed, and another photoresist layer is formed to define the gate area. Etching is conducted to form a trench 14 in the substrate 10. After removing the photoresist layer and before forming a gate oxide layer 16, a sacrificial layer is formed on the surface of the trench with an oxidation process conducted in a furnace. The sacrificial layer is removed. The gate oxide layer 16 is formed and a polysilicon layer acting as a gate 18 is formed in the trench. A patterned photoresist layer is then formed to define source/drain regions, and an implantation and RTA are sequentially performed to form the N+type source 20 at one side of the gate 18. A BPSG layer 22 is deposited with a contact window therein, and P type dopants are implanted to form a P+ type region 24.
The oxidation process of the sacrificial oxide layer, formed before forming the gate oxide layer 16, is given here.
The main oxidation steps are steps 6xcx9c8 after the furnace temperature reaches the oxidation temperature. In these oxidation steps, the flow rate of O2 increases and reaches 12 sccm, and the flow of N2 is interrupted. However, after analyzing the electric properties of the power IC, it is found that the threshold voltage of the power IC is too low and the difference of standard error is very large. This is because the boron is outgassing in these main oxidation steps and the impurities in the environment diffuse into the sacrificial oxide layer to induce dislocation in the substrate. Therefore, the concentration of boron near the surface of the trench is non-uniform. Furthermore, the location of the wafer in the furnace affects the threshold voltage of the power ICs so that the threshold voltage of the power ICs cannot be effectively controlled in the demanded range, so the power ICs belong to a different batch.
Embodiments of the present invention are directed to a method of reducing boron outgassing at trench power IC""s oxidation process for the sacrificial oxide layer whereby the threshold voltage of the power ICs can be improved and the yield of the product can be enhanced.
It is another feature of the present invention to prevent the impurities in the environment from diffusing into the sacrificial oxide layer, and the dislocation in the substrate can thus be reduced.
Yet another feature of the invention is to provide a new oxidation process for sacrificial oxide layer, so the threshold voltage of the power ICs can be controlled in the demanded range, no matter to which batch the power ICs belong and no matter in which zone the wafer is located in the furnace.
An aspect of the present invention is directed to a method of reducing boron outgassing at a trench of a trenched substrate of a power IC during an oxidation process for a sacrificial oxide layer. The method comprises placing the trenched substrate in a furnace; raising a temperature of the furnace to the oxidation temperature; and introducing nitrogen and oxygen into the furnace for main oxidation when the temperature of the furnace reaches the oxidation temperature. The nitrogen is sufficient in concentration in the furnace at the oxidation temperature to reduce boron outgassing at the trench.
In some embodiments, the oxidation temperature is about 1150xc2x0 C. The nitrogen comprises N2. The flow rate of the nitrogen is about 8-20 sccm. Transdichloroethylene (TLC) is introduced into the furnace during the main oxidation after the temperature in the furnace reaches the oxidation temperature. The TLC is introduced after flow rates of the nitrogen and oxygen for the main oxidation are stable. For example, the TLC may be introduced at about 5 minutes after the nitrogen and oxygen for the main oxidation are introduced into the furnace to reach stable flow rates.
In specific embodiments, prior to raising the temperature of the furnace to the oxidation temperature, the furnace is prepared by introducing nitrogen into the furnace at a preparation flow rate which is sufficiently high to make the furnace near a saturated vapor pressure. The temperature of the furnace is ramped up to the oxidation temperature after preparing the furnace. After the temperature in the furnace is ramped up to the oxidation temperature and prior to introducing the nitrogen and oxygen into the furnace for main oxidation, the furnace is stabilized by maintaining the flow rate of the nitrogen at the preparation flow rates. Preparing the furnace may comprise introducing oxygen into the furnace at a preparation flow rate which is substantially lower than the flow rate of the oxygen during the main oxidation. Stabilizing the furnace may comprise maintaining the flow rate of the oxygen at the preparation flow rate. The preparation flow rate of the nitrogen may be about 12-18 sccm. A silicon nitride layer is formed during the main oxidation.
Another aspect of the present invention is directed to a method of reducing boron outgassing at a trench of a trenched substrate of a power IC during an oxidation process for a sacrificial oxide layer. The method comprises placing the trenched substrate in a furnace; preparing the furnace by introducing nitrogen into the furnace at a preparation flow rate which is sufficient to make the furnace near a saturated vapor pressure; ramping up a temperature of the furnace to an oxidation temperature while maintaining the flow rate of the nitrogen into the furnace at the preparation flow rate to make the furnace near the saturated vapor pressure; stabilizing the furnace by maintaining temperature of the furnace at the oxidation temperature and maintaining the flow rate of the nitrogen at the preparation flow rate to make the furnace near the saturated vapor pressure; and performing main oxidation of the trenched substrate by introducing nitrogen into the furnace at a main oxidation flow rate and introducing oxygen into the furnace at a sufficient flow rate for main oxidation.
In some embodiments, the preparation flow rate of nitrogen is about 12-18 sccm. The oxidation temperature is about 1150xc2x0 C. The main oxidation flow rate of nitrogen is about 8-20 sccm. The flow rate of oxygen during the main oxidation is about 12 sccm. TLC may be introduced into the furnace during the main oxidation, after the flow rates of the nitrogen and oxygen for main oxidation are stable, until substantially all impurities are trapped. A silicon nitride layer is formed during the main oxidation.
In accordance with another aspect of the invention, a power IC comprises a substrate, and a gate structure overlying a silicon nitride layer formed in a trench of the substrate during main oxidation of the substrate.